Certain vehicles are constructed with suspensions in which one or both axles are constructed so that each vehicle wheel is connected to a long portion that pivots about a point near the centerline of the undercarriage. In other words, the axle is not a single rod-like member extending across the width of the vehicle with wheels at both ends, but rather two different rod-like members each pivotally extending approximately along half the width of the vehicle. The most notable examples at present include certain passenger trucks manufactured by the Ford Motor Company of Dearborn, Mich. This type of suspension is called a twin I-beam suspension.
Like all suspensions, twin I-beam suspensions must be in proper alignment for the vehicle to operate safely and efficiently. Vehicle service technicians must be able to align such suspensions. There are, however, added complications to correction of alignment angles in twin I-beam suspensions. Such suspensions contain a removable member of roughly ovoid shape (sometimes known as an offset bushing, bushing or cam insert) that must be of a specific size and angular orientation with relation to the I-beam. The placement of the bushing of a particular size in a particular angular orientation affects two important suspension angles: camber and caster. The placement of the bushing simultaneously affects both camber and caster.
When a twin I-beam suspension is out of alignment, it is sometimes necessary to remove the bushing and replace it with a different one. Care must be taken in the selection and insertion of a replacement bushing. For example, if caster is out of alignment but camber needs no adjustment, one cannot simply select a bushing that corrects for caster. Fixing caster while ignoring camber would then put camber out of alignment.
Twin I-beam vehicles are supplied at the factory with a factory installed bushing. Such OEM bushings are constructed not to rotate. They are of a particular size (sometimes known as "offset") and are installed at a particular angle that does not change. Since they do not rotate, they automatically correspond to a particular caster and camber rating, which are conveniently stamped onto the bushing. These numbers correspond to the addition to measured caster angle and to measured camber angle, respectively, that they cause when inserted into a suspension. While the factory installed bushings might not change, general wear and use of the vehicle can create misalignments in the angles they affect. In such cases, it is only by proper replacement of such OEM bushings by an aftermarket bushing that camber and caster may be adjusted.
Aftermarket bushings differ from OEM bushings. They are rated for a particular size (offset) but are not rated for particular camber and caster angles. The addition to camber and caster that their insertion causes is determined by the angular orientation to which they are turned by insertion. Hence, to correct camber and caster alignment in a twin I-beam suspension vehicle, a vehicle technician is faced with two different angles (camber and caster) each affected by two different variables within potential replacement parts (offset and angular orientation). This can be confusing.
It has been common for vehicle technicians to choose the size and orientation of replacement bushings by trial and error. This is time consuming, leads to substantial possibility for error, and results in frustration to the service technician. Some vehicle repair establishments do not perform alignment adjustment on twin I-beam vehicles because of the perceived and actual difficulty of the procedure, and the substantial possibility of returning the vehicle to its owner in worse shape than it came.
Another problem is when a subsequent angle adjustment is necessary. In this case, the twin I-beam vehicle no longer has a factory installed bushing, but rather has a previously inserted aftermarket bushing. As mentioned, aftermarket bushings do not have camber and caster ratings stamped on them. Thus the service technician must determine, prior to commencing the alignment angle adjustment, the actual value of camber and caster that is measured that is attributable to the previously inserted aftermarket bushing. Only then may the technician meaningfully determine from the camber and caster measured values and specification values the necessary rating (offset and angular orientation) of any subsequent correcting aftermarket bushing.
What is needed is automotive service equipment that assists service technicians in the selection of the size and angular orientation of replacement bushings. Further, what is needed is automotive service equipment that assists in replacement of previous aftermarket bushings.